1. Field of the Invention
The present invention relates to a CMOS image sensor, and more particularly, to an improved CMOS image sensor that prevents dark current from occurring and a method of fabricating the same.
2. Discussion of the Related Art
Generally, an image sensor is a semiconductor device that transforms an optical image into an electric signal. Image sensors are mainly classified into charge coupled device image sensor (hereinafter abbreviated as “CCD”) and complementary metal oxide semiconductor image sensor (hereinafter abbreviated as “CMOS”).
CCDs consist of a plurality of photodiodes (PD) arranged in a matrix, a plurality of vertical charge coupled devices (VCCD) provided in vertical direction between the plurality of the photodiodes, a horizontal charge coupled device (HCCD), and a sense amplifier. The photodiodes convert optical signals into electric signals by outputting charges that are transferred in the vertical direction by the VCCDs. The charges transferred by each VCCD are then transferred in a horizontal direction by the HCCD. Finally, these charges in the horizontal direction are sensed by the sense amplifier that in turn outputs the electric signal.
The above-configured CCDs have a complicated driving mechanism, they require considerable power consumption, and they need a complicated fabricating process with a lot of photo-processes. Additionally, it is disadvantageous to use the above configured CCDs when trying to reduce the size of a product because it is difficult to integrate a control circuit, a signal processing circuit, an analog/digital converting circuit (A/D converter) and the like on a CCD chip.
Recently, focus has turned to CMOS image sensors as the next generation image sensors because they overcome the disadvantages of CCDs. The CMOS image sensor employs a switching method for sequentially detecting an output of each unit pixel by means of MOS transistors. The MOS transistors are formed on a semiconductor substrate. Each MOS transistor corresponds to a unit pixel. Additionally, a control circuit, signal processing circuit and the like are used as peripheral circuits. The CMOS image sensor having a photodiode and a MOS transistor within a unit pixel implements an image by sequentially detecting an electric signal of each unit pixel according to a switching method.
The CMOS image sensor, is advantageous because of its low power consumption, and because it requires a simple fabrication process with a small number of photo-processing steps. Additionally, it is possible to integrate a control circuit, a signal processing circuit, an analog/digital converting circuit and the like on a CMOS sensor chip, thereby facilitating the miniaturization of a product. Accordingly, the CMOS image sensor may be widely used for various applications such as digital still cameras, digital video cameras and the like.
CMOS image sensors are classified into 3-T type, 4-T type, 5-T type and the like according to the number of transistors. The 3-T type CMOS image sensor consists of one photodiode and three transistors. And, the 4-T type CMOS image sensor consists of one photodiode and four transistors.
FIG. 1 is a layout of a unit pixel of a general 3-T CMOS sensor.
Referring to FIG. 1, in a unit pixel of a general 3T type CMOS image sensor, an active area 10 is defined. A photodiode 20 is formed on a wider part of the active area 10. Gate electrodes 120, 130 and 140 of the three transistors are formed to overlap the rest of the active area 10. The three transistors, namely, the reset, drive and selection transistors Rx, Dx and Sx are configured with the gate electrodes 120, 130 and 140, respectively. Impurity ions are implanted into the active area 10 but not below the gate electrodes 120, 130 and 140. The impurity ions form source/drain regions for each of the three transistors. Thereafter, a power voltage Vdd is applied to the source/drain regions between the reset and drive transistors, Rx and Dx, while the source/drain regions of the select transistor Sx are connected to a read circuit (not shown in the drawing).
Each of the gate electrodes 120, 130 and 140 is connected to a corresponding signal line (not shown in the drawing). Each of the corresponding signal line is provided with a pad on one end thereof so that it may be connected to an external drive circuit.
FIG. 2 is a cross-sectional diagram according to the line II-II′ of FIG. 1 of a 3T type CMOS image sensor according to the prior art, showing a photodiode and a reset transistor.
As shown in FIG. 2, a P−-type EPI layer 101 is formed on a P++-type semiconductor substrate 100. The semiconductor substrate 100 is defined by an active area (10 in FIG. 1) and a device isolation area. An isolation layer 103 is formed in the device-isolation area.
A gate oxide 121 is formed on the EPI layer 101. A gate 123 is then formed on the gate oxide 121, thereby forming a reset transistor. Dielectric spacers 125 are formed on both sidewalls of gate 123.
N−-type diffusion area 131 and Po-type diffusion area 132 are formed sequentially in the EPI layer corresponding to a photodiode (PD). In this example, the Po-type diffusion area 132 is formed on the N−-type diffusion area 131. Additionally, a high-density n-type diffusion area (n+) and a low-density n-type diffusion area (n−) are formed as source/drain (S/D) area for the reset transistor.
The conventional CMOS image sensor of the structure described above, has the disadvantage of resulting in increased dark current, which causes deterioration of the charge-storing capability and thus of the general performance of a device.
The dark current is created by electrons transferring from the photodiode to other areas of the device when no light enters the photodiode. It has been reported that the dark current is due to the dangling bond or defects distributed around the surface-adjacent area of the device, the boundary area between the isolation layer 103 and the Po-type diffusion area 132, the boundary area between the isolation layer 103 and the N−-type diffusion area 131, the boundary area between the Po-type diffusion area 132 and the N−-type diffusion area 131, Po-type diffusion area 132 and N−-type diffusion area 131. The dark current may cause serious problems such as deterioration of charge-storing capability and general deterioration in performance of the CMOS image sensor in low illumination circumstances.
To reduce the dark current effect occurring at the surface-adjacent area of the device, according to the conventional CMOS image sensor, the Po-type diffusion area 132 is formed on the N−-type diffusion area 131 for the photodiode. Such CMOS image sensor, however, is seriously affected by the dark current occurring at the boundary area between the isolation layer 103 and the Po-type diffusion area 132 and the boundary area between the isolation layer 103 and the N−-type diffusion area 131.
Furthermore, as shown in FIG. 2, when a photoresist pattern is formed on the EPI layer 101 as an ion injection mask to form the N−-type diffusion area 131 and Po-type diffusion area 132 for the photodiode, the whole active area corresponding to the photodiode is exposed through the opening made in the photoresist pattern. When impurities are injected to the exposed active area using ion injection process to form the N−-type diffusion area 131 and Po-type diffusion area 132, they are also injected into the boundary area between the active area 131 and 132 of the photodiode and the isolation layer 103. These ions induce damages and defects at the boundary area between the active area 131 and 132 of the photodiode and the isolation layer 103. These defects produce electrons and hole carriers, and make the electrons recombine. Consequently, leakage current of the photodiode increases and so does the dark current of the CMOS image sensor.
Consequently, because it is hard to inhibit the dark current occurring at the boundary area between the isolation layer 103 and the active area 131 and 132 of the photodiode there is a limit to the improvement of dark current characteristics in a conventional CMOS image sensor.